Oxygen compressors run the risk of fire in the event that there is a rub or contact between a rotating part and a stationary part. In the presence of pure, high pressure oxygen, the heat generated by a rub can start combustion of most ordinary metals, including iron, steel, stainless steel, aluminum, titanium and bronze. One approach considered by the prior art is to line the shroud of an oxygen compressor with leaded bronze, and design for a higher than normal clearance between the stainless steel impeller and the bronze. However, tests indicate that in the event of a rub, stainless steel rubbing on porous silver provides a higher safety margin than stainless steel rubbing on bronze.
Silver is known to be one of the best materials for the stationary parts of oxygen compressors, but the prior art has not taught a reliable way to provide a silver surface on the inner surface of a shroud. Porosity can be achieved by the addition of plastic powder (fugitive) to the metal powder prior to spraying. In this the fugitive process, however, there is no way to insure that all the plastic is removed without changing the favorable mechanical properties of the metal.
The following prior art patents relate to porous and/or abradable materials which are used with motors and compressors:
U.S. Pat. No. 4,037,998 to Goloff is directed to an improved rotary engine which utilizes a thin wear resistant metal layer located in sealing engagement with the rotor as the motor moves within the chamber. A backing for the thin layer is formed of a metal having a high thermal conductivity, and is provided with a plurality of relatively closely spaced cooling passages to provide for an improved cooling structure for the engine to thereby extend its life. The metals taught include copper, brass, aluminum and magnesium.
U.S. Pat. No. 4,056,339 to Doi et al. is directed to a rotary piston type internal combustion engine in which the rotor housing is plated with a pin-point porous chromium plating having a porosity in the range of 10-60% with a certain specified hardness. Tile porosity appears to be used for its oil retaining properties.
U.S. Pat. No. 4,207,024 to Bill et al. is directed to a composite seal for turbo machinery in which the shroud contains a compliant backing and where the compliant material of the backing is covered with a thin ductile layer. The thin layer may be a metal or metal alloy layer formed from a dense plasma sprayed soft metal such as aluminum or bonded metal sheath or foil.
U.S. Pat. No. 4,867,639 to Strangman is directed to abradable shroud coatings that are applied to a turbine or compressor shroud structure to facilitate reduction in blade tip-to-shroud clearance for improved engine performance or airfoil durability. The coating may include soft burnishable ceramic material such as CaF.sub.2 or BaF.sub.2 in a ceramic or a metallic matrix or honeycomb structure.
An objective of the present invention is to provide for an abradable, non-sparking metallic layer that is easily abraded in the event of contact with another metal, and which overcomes the problems and limitations of the prior art described above.